The invention relates to a device for encapsulating blanks of high-temperature metallic alloys, especially TiAl alloys, which are subjected to a forging or rolling process for hot forming.
Metallic, high-temperature alloys are used for the manufacture of highly strained or highly stressed components such as turbine components for use in airplane propulsion turbines. In order to achieve the desired properties, such as high strength, it is for certain components basically necessary that they have been hot-formed. In the case of TiAl alloys as the metallic high temperature alloy, hot forming of the components is necessary also with regard to obtaining a certain grain structure which could not be achieved in any other way, that is, by melt metallurgy. It has been found that the hot forming of TiAl casting blocks requires temperatures of 1100xc2x0 C., see Y. -W. Kim, D. M. Dimiduk, J. Metals 43 (1991) 40. This however is possible only in a non-isotherm manner, for example during forging or rolling, because of the temperature limits provided by the mold or receiver structures. Since the malleability and form resistance of TiAl alloys are highly temperature dependent, the blanks need to be encapsulated for the forging or rolling procedure in order to avoid high temperature losses. As encapsulating materials, Ti-alloys or austenitic steels are available whose form-change resistance however is, at the required temperatures, very much smaller than that of TiAl blanks or respectively, an unfinished body consisting of that material. The use of encapsulating materials with a better adapted forming resistance such T2M-molydenum is not reasonable for cost reasons.
The large differences in the forming resistances of the encapsulating and the core materials leads during forging or rolling to non-uniform shaping with undesirable variations in the degree of the shape over the length of the strand and furthermore to the formation of cracks in the capsules. It has been tried to adapt the forming resistances between the capsule and core materials to one another by providing a cooling phase between the heating and the strand pressing steps. Computer models of the temperature curve of capsule and core with an increasing pause show that the temperature differences achieved in this way are too small.
Also, with a low assumed heat transfer value as it can be achieved only with a heat insulation layer (for example, glass wool), the temperature difference achievable is still not sufficient.
It is therefore the object of the present invention to provide a device for encapsulating blanks of metallic high-temperature alloys, whereby heat losses of the blank are avoided. In accordance with the object, the encapsulation is cooled by increased waiting periods between the heating and the forging or rolling procedure at low temperature losses in the core to such a degree that the encapsulation material and the core material have almost the same forming resistance for which temperature differences of up to 500xc2x0 C. are necessary. The device should be simple and inexpensive.
In a device for encapsulating blanks of metallic high temperature alloys, particularly TiAL alloys, which are subjected to forging or rolling for not forming, at least a first inner envelope surrounds the blank in closely spaced relationship and a second envelope surrounds the first envelope and both envelopes consist of a metallic material.
With such a device, heat radiation out of the blank, that is out of the core of the arrangement, is minimized. At the given temperatures, the heat radiation is the largest cause for the heat losses. It is possible furthermore to provide for minimal heat conductivity by vacuum insulation, whereby also heat transfer by convection is avoided. Also, material combinations are avoided. With this type of forging or rolling at the required high temperatures, undesired reactions would otherwise occur.
It has been found that, in order to form an effective radiation shield for the inner envelope, a sheet metal structure is sufficient to reduce the heat energy radiated off the blank by 33%.
The outer envelope of the device should preferably have a wall thickness of 5 to 10 mm as tests have shown. Basically, the outer envelope consists of steel or preferably of a titanium alloy such as TiAl6V4.
Tests have further shown that the inner envelope should preferably have a wall thickness of only 0.1 to 1 mm. A wall thickness of 0.3 mm was found to be particularly advantageous in order to achieve a reduction of the heat radiation by 33%. Because of the high heating and working temperature on one hand and because of costs on the other, the inner envelope preferably consists of foils of molybdenum and/or tantalum, which have low heat emission characteristics. In this way also, material combinations are avoided which would lead to undesired reactions at the high temperatures required.
In principle, it is possible in different ways, to ensure that there is always a gap between the blank and the surrounding envelope in order to avoid heat contact between the blank and the inner envelope. But it has been found to be advantageous to shape the blank such that it has a plurality of projecting webs which act as spacing members between the blank and the surrounding envelope. If the blank is essentially cylindrical, the webs can be formed in a simple manner by turning or cutting.
In order to make sure in the same manner as described earlier that the inner envelope is only in a negligible heat contact with the outer envelope, the outer envelope may have a plurality of inwardly projecting webs which are directed toward the inner envelope and which act as spacers for the inner envelope. Also, these webs may, in principle, be formed by turning or suitably cutting them from the outer envelope particularly if the outer envelope has a hollow cylindrical shape. The webs of the outer envelope and of the inner blank or the core are preferably so formed that their contact areas with the adjacent inner envelope is small relative to the rest of the outer surface area.
As already mentioned, a single inner envelope serving as a radiation shield may reduce the heat radiation by 33%. In order to further reduce the heat radiation from the blank, a third and a fourth envelope may be disposed between the first and the second envelope in closely spaced relationship. The selection of additional envelopes depends on whether it is considered necessary to provide the same forming resistance for the envelope and the core material for a particular forging or rolling procedure dependent on the material forming the blank.
As with the basic arrangement as described above, wherein at least two envelopes are provided, it may also be advantageous for an arrangement with four envelopes to provide the third envelope adjacent the first inner envelope with a plurality of webs projecting toward the first and the fourth envelope so as to form spacers with respect to the first and fourth envelope. Also in this case, the webs can be formed by turning or cutting of the third envelope. The blank and the outer envelope would still be turned or cut to form the webs thereon as described earlier.
Preferably, the third envelope consists of the same material as the second envelope and preferably the fourth envelope consists of the same material as the first envelope.
Altogether, with a device made in this way with four envelopes, the energy radiated from this blank is reduced to 25%.
Finally, for a device with two envelopes or more envelopes, the outer envelope must be vacuum tight so that heat transfer through the gas in the spaces between the envelopes as well as heat transfer by convection of gases in the spaces between the envelopes is suppressed. Furthermore, an oxidation of the metallic parts is prevented in order to maintain the low emission capabilities of these parts.
The invention will be described below with reference to the accompanying schematic drawings and graphic representations on the basis of a particular embodiment and some modifications thereof.